A Handoff Candidate List consists of a list of a plurality of cellular base stations (such as cells 32-34 of FIG. 3) to which a mobile transceiver may be handed-off when leaving the service region of the source cell 31. The list of potential handoff candidates for each cell is used by a switch to locate another cell which is better able to serve the mobile.
Generation of the handoff candidate lists for an entire cellular system requires knowledge of how Radio Frequency (RF) propagation from each cell interacts with the propagation of its surrounding cells.
The creation of a handoff candidate list is, in general, a subjective process. Equally skilled engineers, given the same set of input parameters, would be unlikely to produce identical handoff candidate lists. A common methodology involves producing a best server map which indicates the primary coverage regions of each cell, and designating handoffs between cells whose coverage regions share a border (e.g. cells 31 and 33). Potential candidates are eliminated by visual inspection when the line-of-sight path between the source and target cell site is obstructed or when that path crosses through a significant portion of a third cell site. This manual process is time consuming and is prone to error, especially when a large number of cell sites are involved.
When determining which cells are preferred handoff candidate cells, distance and direction are important factors to consider. But distance and direction alone are not always sufficient criteria. This is because the terrain (buildings, trees, etc.) affects how the signals travel so that the signal strength at the same distance from a cell varies with the direction. Therefore, it is important to determine how the terrain affects the propagation.
For the purposes of this description, the terms "relative signal strength" and "signal-to-noise value" are used interchangeably, although more precisely, the relative signal strength values represent potential signal-to-noise, since the transmitting frequency is not taken into consideration. A signal-to-noise matrix is developed to represent how much the signal from one cell would interfere with the signal from another cell if the two cells were to transmit on the same frequency. A low signal-to-noise value tends to indicate cells which are transmitting in close "RF proximity" and as such, are likely to require the ability to handoff to one another.
The signal-to-noise matrix is actually based on the relative amount of area which is common to the serving areas of both cells. The serving area of a cell is the region where a mobile would be serviced by the cell, or sector of a cell in a sectorized design. This is defined by a set of all the points where a signal from the serving cell has the strongest signal strength compared to all other cells transmitting signals. This attempts to model what occurs when a mobile is "handed off" from one cell to another cell. When the signal strength from another cell becomes stronger than the signal strength from the cell to which the mobile is currently linked, the mobile is "handed off" to the cell providing the stronger signal.
In actuality, a handoff does not occur when a signal is detected from another cell which is just slightly stronger than the current signal. This would cause a hysteresis effect where the mobile is constantly oscillating between two cells with similar signal strength values. This problem is overcome differently in analog and digital systems. In an analog system, a handoff occurs when the current signal drops below some predetermined minimum threshold. In a digital system, the handoff occurs when another signal is detected which is stronger than the current signal by some predetermined threshold.
The signal-to-noise matrix is computed by averaging the signal-to-noise values between a source cell 31 and a target cell 33 for all points where the source cell is a potential server. This is accomplished by comparing the signal strength from the source cell 33 to that of all other cells 32-34 in the cellular system 30. If the source cell 31 has the best signal strength at that point, then it is considered to be the serving cell. If it falls below the best signal strength, then it is not considered the serving cell and a handoff to another cell will occur. In reality, the signal strength must fall below a minimum threshold in the analog case, and below a relative threshold to other carriers in the digital case. So when deciding if the source cell 31 is still the carrier we apply either or both of these criteria. If the signal strength falls below a minimum value, or if it falls outside of a range below the best signal strength, then it is not the carrier at that point and the signal-to-noise (S/N) ratio is not computed.
The method to generate the relative signal-to-noise matrix can be described as follows: for every possible pairing of one cell to another in the cellular system, compare the signal strength at every point common to both cell's propagation regions against the best signal strength at that point for all cells in the system. If the source cell's signal strength at a point is greater than some minimum value and is greater than or equal to the best signal strength at that point less some threshold value, compute the signal-to-noise value at this point. The average of all the signal-to-noise values computed for a source and target cell pair is the signal-to-noise value for that pair.